The present invention relates generally to circuit breakers. More particularly, the present invention is directed to a circuit and method for measuring the switch opening time of a medium voltage air circuit breaker.
Electrical power distribution systems and their components need protection from numerous types of malfunctions, including overcurrent conditions, overvoltage conditions, undervoltage conditions, reverse current flow, and unbalanced phase voltages. Accordingly, electrical distribution systems are provided with circuit breakers, which open to break the circuit a prescribed time after detection of a malfunction condition. The integrity of operability of any power distribution system ultimately depends on the proper functioning of the circuit breakers.
Circuit breakers are mechanical switching devices capable of making, carrying, and breaking currents under normal circuit conditions and also making, carrying for a specified time, and breaking currents under specified abnormal conditions such as those described above. The medium in which circuit interruption is performed may be designated by a suitable prefix, for example, air-blast circuit breaker, gas circuit breaker, oil circuit breaker, or vacuum circuit breaker.
Circuit breakers are rated by voltage, insulation level, current interrupting capabilities, transient recovery voltage, interruption time, and trip delay.
Regardless of the medium of arc quenching and insulation, each circuit breaker unit consists of the following construction elements: (1) main contact at system voltage, (2) insulation between main contact and ground potential (porcelain, oil, gas), (3) operating and supervisory devices as well as accessories out of reach of the system voltage-life zone, and (4) an insulated link between operating device and main contact.
In operation, a circuit breaker is opened by energizing a trip coil associated with the circuit breaker. Once the trip coil is energized, the main contacts are opened and then auxiliary contacts are opened to prevent damage to the trip coil.
A typical medium voltage (e.g., 15,000 volts) air circuit breaker, operating properly, opens in approximately 3-xc2xd cycles, measured from the energizing of the circuit breaker trip coil until distinguishing of the arc between the main contacts and the system. The same breaker, if in need of repair, can take as long as 12 cycles to achieve circuit interruption. Typical circuit breakers can remain closed and in continuous service for many years. Due to the number of mechanical parts associated with a typical circuit breaker, circuit breakers often xe2x80x9cstickxe2x80x9d if the circuit breaker remains closed for an extended period of time. Lubricants do not adequately remedy this problem. Therefore, it is desirable to measure the circuit breaker""s opening time the first time it is opened, and to remove the circuit breaker if it is defective (i.e., if the breaker does not open within a prescribed period of time).
Conventionally, circuit breakers are serviced periodically (e.g., once a year) by shutting down some portion of the power distribution system relatively long periods of time (e.g., hours). For complex power distribution system, it can be difficult to implement such a service program.
Therefore, it would be desirable to economically implement a circuit and method for measuring the opening time of a circuit breaker which minimizes disruption of the operation of the power distribution system.
The present invention overcomes the above-described problems, and provides additional advantages, by providing for a method and circuit for measuring the opening time of a circuit breaker which minimizes down time of the power distribution system and which is not connected to the main power contacts of the circuit breaker. The circuit is powered by the existing substation battery system. A sensing circuit senses a DC current energizing the trip coil in the circuit breaker and starts a timer a programmable time period after the current is sensed. The timer stops when the DC current stops flowing in the breaker trip coil circuit to measure the interruption time of the circuit breaker. The sensing means of the timing circuit includes a current transformer-type sensor, through which a trip coil circuit conductor passes.
The circuit according to the present invention advantageously does not require connection to the main power contacts of the circuit breaker, and therefore minimizes shut down time of the power distribution system to measure the switch time of the circuit breakers. According to the disclosed embodiment, the opening time can be measured within approximately 30 seconds.